The invention relates to a decoder and decoding method for decoding serially-received packetized data, and, in particular, for decoding packetized data that have been coded with an overhead sufficiently low to enable serial transmission through an Ethernet local area network with a bit rate of 10 gigabits/second (Gb/s) using an OC192 SONET laser operating at 10.3 Gb/s.
For several decades now, integrated circuit and laser technologies have doubled in performance approximately every 18 months. These technologies have been used to support a rapidly-growing demand for global communications capacity. This demand is currently growing much faster than the underlying rate of improvement of the supporting technologies. As an example, communication traffic through the Internet has recently been doubling every nine months. The demand for additional current bandwidth is severely stressing the capabilities of current electronic and optical technologies.
In particular, the Ethernet local area network standard has progressively increased in speed by factors of ten, starting at 10 megabit per second (Mb/s) in 1982. Proposals for a 10 gigabit/second (Gb/s) Ethernet standard were made in 1999. The most recently adopted Ethernet standard used a 8b/10b line code described by A. X. Widmer and P. A. Franaszek in A DC-Balanced, Partitioned-Block, 8b/10b Transmission Code, 27 IBM J. Res. AND Dev., (1983 September) for transmitting serial data at 1 Gb/s. In 8b/10b line code, each eight-bit input word is represented by a ten-bit code that is transmitted on the data link. In exchange for this 25% overhead, 8b/10b coding provides DC balance, and a guaranteed transition density. The ten-bit code additionally has the ability to represent an assortment of control words used for signalling and framing.
Re-using 8b/10b coding for sending information at 10 Gb/s was considered in the proposed 10 Gb/s Ethernet standard. However, using this. technique would result in having to transmit at a baud rate of 12.5 Gbaud, i.e., 12.5 Gb/s.
With currently-available laser fabrication technology, manufacturing a laser capable of modulation at 12.5 Gb/s at a modest price is considered to be quite difficult. However, laser systems currently exist for use in systems conforming to the OC-192 SONET telecommunications standard. Such system operate at signalling rates of 9.95328 Gb/s. However, these commercially-available lasers do not have enough performance margin to run at more than 25% faster than their design speed.
One way to enable the lasers designed for use in SONET telecommunications systems to be used in the proposed 10 Gb/s Ethernet standard would be to design a simple and robust coding scheme with a lower overhead than 8b/10b line code. In principle, this goal can be achieved using a block code in which words of M bits are represented by an N-bit code and in which the ratio of N:M is less than 10:8.
A potential coding scheme having a lower overhead than 8b/10b line code is that used in the SONET telecommunications standard. The SONET coding scheme assures DC balance by using a scrambling system, and has an overhead of about 3%. However, the scrambling system used in the SONET coding scheme uses two layers of polynomial scrambling to achieve an adequate level of protection. This two-layer scheme is complex to implement. Moreover, the SONET coding scheme has a complex framing protocol that is difficult to implement at low cost. The SONET coding scheme would also have to be modified to add an extra level of encoding to support Ethernet packet delimiting. Such an extra level of coding would probably increase the overhead of the SONET coding to 7% or more. In addition, it is thought that the networking community would find the wholesale adoption of a telecommunications standard to be unpalatable. The performance and political difficulties just described would make it difficult for a standard based on the SONET coding scheme to be adopted as a new Ethernet standard.
Another potential coding scheme having a lower overhead than 8b/10b line code is that known as CIMT. This coding scheme is described in U.S. Pat. No. 5,022,051 of Crandall et al. and U.S. Pat. No. 5,438,621 of Hornak et al. The CIMT code is an (M)b/(M+4)b code that can be configured have a lower overhead than 8b/10b line code by making the value of M sufficiently large. However, for large values of M, the CIMT code is difficult to implement due to the need to compute the DC balance of an incoming block of M bits, and the need to compute a running DC balance of the transmitted bits in real time.
In a patent application filed simultaneously with this disclosure, the inventors disclose a new coder and coding method that employ a novel 64b/66b coding scheme to provide serial data transmission through an Ethernet local area network with a bit rate of 10 gigabits/second (Gb/s) using an OC192 SONET laser operating at 10.3 Gb/s.
What is additionally required is a decoder and decoding method for efficiently and reliably decoding the frames of serially-received data that have been coded with the 64b/66b coding scheme. What also is needed is a decoder in which the integrated circuit die size and power dissipation are minimized and a decoding method which can be embodied in data receivers in which the integrated circuit die size and power dissipation are minimized. The decoder and decoding method should also meet the performance requirements of the new Ethernet standard with respect to error detection.
The method provides a method for decoding a frame of data. The frame is one of a set of frames that represent a packet of information words and that additionally represent coded control words preceding and following the packet. The frames each include a master transition and a payload field. The payload field either is composed exclusively of ones of the information words, or includes a TYPE word that identifies the structure of the payload field. The master transition is in a first state when the payload field is composed exclusively of ones of the information words, and is otherwise in a second state. In the method, a determination is made of whether the master transition is in the first state.
When the master transition is in the first state, the payload field is adopted as a block of received data.
When the master transition is not in the first state, the TYPE word is extracted from the payload field, the payload field is expanded in response to the TYPE word, and the payload field, after expansion, is adopted as a block of received data.
The invention additionally provides a decoder for decoding a frame of data. The frame is one of a set of frames that represent a packet of information words and that additionally represent coded control words preceding and following the packet. Each frame includes a master transition and a payload field. The payload field either is composed exclusively of ones of the information words, or includes a TYPE word that identifies the structure of the payload field. The master transition is in a first state when the payload field is composed exclusively of ones of the information words, and is otherwise in a second state. The decoder comprises a frame decoder, a type word extractor and a block generator.
The frame decoder receives the frame and separates the frame into the master transition and the payload field.
The type word extractor is connected to receive the payload field and the master transition from the frame decoder and operates only when the master transition is in the second state to extract the TYPE word from the payload field.
The block generator is connected to receive the payload field, the TYPE word and the master transition. When the master transition is in the first state, the block generator operates to adopt the payload field as a block of received data.
When the master transition is in the second state, the block generator operates to expand the payload field in response to the TYPE word, and to adopt the payload field, after expansion, as the block of received data.
The decoder and decoding method according to the invention are capable of decoding frames of data that have been coded with a very low overhead when the coding is implemented as a 64b/66b code (3.125%). The overhead is substantially lower than 8b/10b (25%). The decoder and decoding method according to the invention allow Ethernet data to be transmitted at a bit rate of 10.0 Gb/s using existing lasers designed for use in SONET OC-192 transmitters. A 10 Gb/s Ethernet standard that employs the decoder and decoding method according to the invention can be adopted now rather than having to wait for lasers capable of modulation at 12.5 Gbaud to be developed.